Not Applicable.
Not Applicable.
The present invention relates to the field of particle counting in gases, particularly reactive, corrosive gases for the electronics fabrication industry.
Minute amounts of contamination can adversely affect the microchip fabrication process in the electronics industry. Contamination in the form of particles causes short circuits, open circuits, and other defects. These defects can cause finished micro-electronic circuits to fail. Such failures are responsible for significant yield reductions in the micro-electronics industry. Yield reductions caused by micro-contamination substantially increase processing costs.
Micro-electronic circuits require many processing steps. Processing is performed using extremely clean gases. However, the amount of contamination needed to produce fatal defects in micro-circuits is extremely small. For example, an individual particle as small as 0.02 micrometer in diameter can result in a fatal defect in a modern micro-circuit. Micro-contamination may occur at any time during the many steps needed to complete a circuit. Therefore, tight control of cleanliness in the processing gas is required.
Modern filters are able to remove particulate contaminants in process gases with an extremely high efficiency. However, the complete assurance of contamination control also requires verification of gas cleanliness. An accurate technique for detecting microscopic particles in filtered gases must be available. This technique must be capable of detecting microscopic contaminant particles as small as 0.02 micrometer in reactive or toxic gases used in microchip processing, including but not limited to the following gases: SiCl4, PH3, B2H6, AsH3, SiF4, Si2H6, NH3, BCl3, BF3, Cl2, H2, HBr, HCl, HF, NF3, N2O, O2, SiH4 and WF6, as well as inert gases, including, but not limited to such gases as N2, Ar, He, CF4, CHF3, C2F6 and SF6.
Ideally, the technique should perform the particle measurement at a pressure equal to the process gas system line pressure (about 60 psig). By performing the measurement at the gas line pressure, the sample gas would not need to be reduced in pressure before entering the measuring instrument. That is, the instrument could be connected directly to the gas line without intermediate pressure reducing devices. Measurement at gas line pressure would provide an advantage in particle measurement, since the process of pressure reduction in some gases can produce adverse effects such as particle shear-off or nucleation of impurities in the pressure reduction device. Such objects produced by shear-off or nucleation would be falsely interpreted by the downstream particle counting instrument as gas line contaminant particles.
Also, many pressure reduction devices require venting a portion of hazardous and expensive gases before the remaining sample can enter the instrument. Such venting is costly, environmentally damaging, and may require increased flow capacities through vent system emission control devices.
Finally, condensation of the process gas into liquid droplets may occur during the process of pressure reduction, especially when sampling high boiling point and easily condensed gases such as HF, WF6 and BCl3. Such condensation droplets also would be falsely interpreted by the downstream particle counting instrument as gas line contaminant particles.
It is therefore advantageous to develop a sampling technique that can measure contaminant particles as small as 0.02 micrometer in toxic or reactive microchip processing gases without the need for intermediate gas pressure reduction.
Previous attempts to obtain continuous counting of contaminant particles in reactive or toxic process gases, as well as inert gases, have included laser particle spectrometers or laser particle monitors. These instruments determine the equivalent optical diameters of contaminant particles through a process of light scattering from individual particles. The number of light pulses scattered is equal to the number of particles passing through the optical sensing volume of the instrument. Such instruments have been developed for use with reactive or toxic gases, and for pressurized sample gases. Modern laser particle counters typically function with low background noise for particles larger than 0.1 micrometer, but are noise limited in lower size detection capability because of light scattering from sub-range particles and gas molecules. Consequently, such instruments cannot detect contaminant particles smaller than 0.1 micrometer.
Previous attempts to obtain low noise particle detection below 0.1 micrometer have included condensation nucleus counters (CNCs). These instruments use continuous conductive cooling, continuous cooling through dilution, or cooling through expansion to create a supersaturated aerosol mixture. Various substances have been used as a saturating medium, including water, alcohol (e.g., butanol) and perfluorinated organic compounds, such as perfluorodimethyldecalin. The fine particles act as nucleation sites for vapor condensation and subsequent droplet growth. Droplets grow to sufficient size to permit detection by conventional light scattering or light absorption techniques with negligible accompanying noise.
Such a CNC has been described in U.S. Pat. No. 4,790,650 wherein a device admits a gaseous flow into a saturator zone and then takes a portion of the flow through a chilled region to condense a working fluid on entrained particles to enlarge the diameter of the particle to facilitate counting by downstream means, such as an optical particle detection device.
Additional descriptions of CNCs are found in the dissertation by M. R. Stolzenburg, particularly Chapter 5, titled xe2x80x9cAn Ultrafine Aerosol Condensation Nucleus Counterxe2x80x9d, and in an article titled xe2x80x9cA Condensation Nucleus Counter Design for Ultrafine Particle Detection Above 3 nm Diameterxe2x80x9d by P. B. Keady, V. L. Denier, G. J. Sero, M. R. Stolzenburg and P. H. McMurry.
U.S. Pat. No. 4,293,217 discloses a continuous flow CNC and process for detecting small contaminants in gas streams. Additional patents pertaining to CNC""s include U.S. Pat. Nos. 3,806,248 and 3,632,210.
The theory and operation of one type of CNC is set forth in an article by M. R. Stolzenburg and P. H. McMurry, entitled xe2x80x9cCounting Efficiency of an Ultrafine Aerosol Condensation Nucleus Counter: Theory and Experimentxe2x80x9d.
The above CNCs were developed for use only with inert sample gases. A CNC designed for use in H2 and O2, as well as inert gases such as N2 and He, was described in an article by A. E. Holmer, M. L. Malczewski, J. Blesener and G. Schurmann entitled xe2x80x9cDesign and Calibration of a Condensation Nucleus Counter Suitable for Use in Hydrogen Servicexe2x80x9d.
A CNC designed for use in H2 and O2, as well as inert gases such as N2, was described in an article by H. T. Sommer, J. R. C. Futrell, L. R. Dominguez-Sommer and D. D. Christman entitled xe2x80x9cCondensation Nucleus Counter Evaluation for Hazardous Semiconductor Process Gasesxe2x80x9d.
An alternative method for measuring particles in reactive gases is disclosed in U.S. Pat. No. 5,231,865. This patent discloses a diffusion gas diluter device and a method wherein a particle-containing reactive gas, such as H2 or O2, is diluted with an inert diluent gas to diminish the reactive characteristics of the particle-containing gas without disturbing the particle concentration of the gas, thus allowing it to be accurately and safely measured for its particle content using a conventional inert gas CNC.
The measurement techniques in the above references are capable in some cases of detecting particles in H2 and O2, and can in some cases detect particles as small as 0.003 micrometer. However, those techniques do not detect particles as gas pressures above 0 psig in toxic or other reactive gases, such as SiCl4, PH3, B2H6, AsH3, SiF4, Si2H6, NH3, BCl3, BF3, Cl2, Hbr, Chl, HF, NF3, N2O, SiH4, and WF6.
Several prior CNCs have been developed for use with an oxidizing gas (O2) or a flammable gas (H2). However, many prior CNCs and their contained working fluids were constructed from materials that are not resistant to reaction with oxidizing or flammable gases. Most prior CNCs were developed for use with air or inert gases. These CNCs were constructed from materials that typically are not resistant to chemical attack from corrosive gases.
The prior CNCs were not constructed to contain gas pressures greater than 0 psig, or to prevent possible leakage of reactive or toxic gases under pressure. Therefore, the prior CNCs require upstream pressure reduction when sampling from pressurized gas sources.
It is desired to have an apparatus and a method for counting and measuring particles substantially smaller than 0.1 micrometer in gases at elevated pressures (i.e., greater than 0 psig), especially toxic and/or reactive gases, including but not limited to such gases used in microchip processing.
It is further desired to have such an apparatus and method which can perform such measurements and counting of particles at pressures substantially equal to process gas system line pressures, typically about 60 psig (i.e., without the need for upstream or intermediate pressure reducing devices).
It is still further desired to have such an apparatus which is constructed from materials that are resistant to corrosion and to reaction with oxidizing or flammable gases, and which can contain elevated pressures (e.g., about 60 psig) and prevent leakage of reactive and/or toxic gases under pressure.
The present invention is an apparatus and a method for detecting particles in a particle-containing gas at a pressure greater than about 0 psig. The invention can detect sub-0.1 micrometer contaminant particles in pressurized reactive or toxic gases, including but not limited to SiCl4, PH3, B2H6, AsH3, SiF4, Si2H6, NH3, BCl3, BF3, Cl2, H2, HBr, HCl, HF, NF3, N2O, O2, SiH4, WF6, and mixtures thereof, as well as inert gases, including but not limited to N2, Ar, He, CF4, CHF3, C2F6, and SF6, and mixtures thereof.
In a preferred embodiment, the apparatus includes a gas distribution line containing a pressurized gas having a pressure greater than about 0 psig and a condensation nucleus counter in fluid communication with the pressurized gas in the gas distribution line. The condensation nucleus counter is adapted to receive a stream of the pressurized gas at a pressure substantially equal to the pressure of the pressurized gas in the gas distribution line.
In another embodiment, the apparatus also includes means for determining the number of at least one particle in the pressurized gas. In yet another embodiment, the apparatus includes means for tabulating the number of the at least one particle. For example, the means for tabulating may be a computer.
In the preferred embodiment, the condensation nucleus counter includes the following: (a) a reservoir block; (b) an inlet tube adapted to receive the stream of the pressurized gas and to deliver said stream into the reservoir block; (c) a saturator disposed inside the reservoir block, wherein the saturator is heated by a heater mounted in thermal contact with the reservoir block; (d) a working fluid disposed inside the saturator; (e) a sintered metal wick partially submerged in the working fluid; (f) a condenser adapted to receive a stream of the pressurized gas from the saturator; (g) an aerodynamic focusing nozzle adapted to receive the stream of the pressurized gas stream containing droplets; (h) an optical detection chamber having a droplet sensing device to count and identify the droplets in the pressurized gas stream containing droplets; (i) and an outlet tube adapted to vent the pressurized gas stream containing droplets from the optical detection chamber.
The condensation nucleus counter is constructed of materials resistant to corrosion and to reaction with any of several pressurized gases, including but not limited to: SiCl4, PH3, B2H6, AsH3, SiF4, Si2H6, NH3, BCl3, BF3, Cl2, H2, HBr, HCl, HF, NF3, N2O, O2, SiH4, WF6, N2, Ar, He, CF4, CHF3, C2F6, and SF6, and mixtures thereof.
Another aspect of the invention is a method of detecting particles in a particle-containing gas at a pressure greater than about 0 psig. The method includes multiple steps as follows: (a) providing a gas distribution line containing a pressurized gas at a pressure greater than about 0 psig; (b) providing a condensation nucleus counter in fluid communication with the pressurized gas in the distribution line, wherein the condensation nucleus counter is adapted to receive a stream of pressurized gas at pressures substantially equal to the pressure of the gas in the pressurized gas distribution line; and (c) introducing a stream of the pressurized gas into the condensation nucleus counter at a pressure substantially equal to the pressure of the pressurized gas in the gas distribution line.
In an alternate embodiment, the method includes the additional step of determining the number of at least one particle in the pressurized gas introduced into the condensation nucleus counter. In yet another embodiment, the method includes the additional step of tabulating the number of the at least one particle. For example, the tabulating step may be performed by a computer.
The step of determining the number of at least one particle in the pressurized gas comprises the following sub-steps: (a) passing a particle-containing gas mixed with a fluid vapor into a condensation zone; (b) condensing the working fluid vapor on at least one particle in the particle-containing gas having a minimum size corresponding to a minimum temperature of a condensing zone to form at least one droplet; and (c) detecting the droplets and counting the number of droplets by appropriate sensing and tabulation.
The working fluid may be perfluorotrimethyllcyclohexane. However, other fluids may be used, including but not limited to non-reactive fluids, such as Multfluor(copyright) perfluorinated hydrocarbons, available from Air Products and Chemicals, Inc. of Allentown, Pa. (Multifluor(copyright) is a registered trademark of Air Products and Chemicals, Inc.).
Using the methods according to the present invention, the particles in a particle-containing pressurized gas can be detected at an at least approximately 50% counting efficiency. A typical pressure of the gas in such a method is about 60 psig. The methods may be used for any of several pressurized gases, including but not limited to: SiCl4, PH3, B2H6, AsH3, SiF4, Si2H6, NH3, BCl3, BF3, Cl2, H2, HBr, HCl, HF, NF3, N2O, O2, SiH4, WF6, N2, Ar, He, CF4, CHF3, C2F6, and SF6, and mixtures thereof.
Another aspect of the invention is an improved gas distribution system containing a pressurized gas having a pressure greater than about 0 psig including a condensation nucleus counter in fluid communication with the pressurized gas distribution system. The condensation nucleus counter is adapted to receive a stream of the pressurized gas at a pressure substantially equal to the pressure of the gas in the gas distribution system.